In recent years there has been increasing interest expressed in the development of microporous membranes of a semipermeable nature which are useful in separating the components of a solution. For instance, semipermeable membranes have been looked to as a possible means to demineralize or purify otherwise unusable water and to thereby alleviate the increasing demands for potable water necessitated by the rapid growth of the population and industry in many parts of the world. Separation techniques which employ such membranes include electrodialysis, reverse osmosis, ultrafiltration, ion exchange, etc.
Electrodialysis separations employ an electrolytic cell having alternating anionic and cationic membranes that collect desalted and concentrated solutions in adjacent compartments. Such a technique can be useful to purify liquids by removing ionizable impurities, to concentrate solutions of electrolytes, or to separate electrolytes from non-electrolytes.
As opposed to the charge dependent types of separations, reverse osmosis utilizes pressure to move materials which may be either ionic or non-ionic selectively through a membrane. Ultrafiltration, which is very similar, uses gravity or applied pressure to effect separation using membranes which act as submicronic sieves to retain large molecules and permit the passage of small, ionic, or non-ionic forms.
The desalination of salt or sea water through the use of semipermeable membranes is commonly characterized by the use of pressure in excess of osmotic pressure and is therefore termed reverse osmosis. The natural tendency for a solution of a higher concentration separated from a solution of lower concentration by a semipermeable membrane, is for the solvent on the side of lower concentration to migrate through the membrane to the solution of higher concentration thereby eventually equilibrating the concentrations of the two solutions. The degree of this natural tendency is termed osmotic pressure. The process may be reversed by applying a force to the side of higher concentration in excess of the osmotic pressure to force the solvent of the solution of higher concentration through the semipermeable membrane to the side of lower concentration thereby bringing about a separation. The natural tendency which is believed to be the result of a difference in free energy resulting from the concentration gradient is observed to operate at a high thermodynamic efficiency, and at ambient temperature.
In the prior art semipermeable membranes have been formed from a variety of natural and synthetic materials including polybenzimidazoles. See, for instance, commonly assigned U.S. Pat. Nos. 3,699,038; 3,720,607; 3,737,042; 3,841,492; and 3,851,025. The polybenzimidazole membranes are recognized to possess superior thermal stability when compared with other semipermeable membranes, such as those formed from cellulose acetate.
It is an object of the present invention to provide a process for beneficially chemically modifying polybenzimidazole membranes.
It is an object of the present invention to provide a process wherein polybenzimidazole semipermeable membranes are chemically modified to render them more flexible and more amenable to bending and handling without deleterious results.
It is an object of the present invention to provide a process wherein polybenzimidazole semipermeable membranes are chemically modified to exhibit improved compaction resistance during high pressure separations which are carried out over an extended period of time.
It is an object of the present invention to provide improved chemically modified polybenzimidazole semipermeable membranes which are insoluble in N,N-dimethylacetamide.
It is an object of the present invention to provide improved chemically modified polybenzimidazole semipermeable membranes which particularly are suited for ultrafiltration separations.
It is an object of the present invention to provide improved chemically modified polybenzimidazole semipermeable membranes which particularly are suited for reverse osmosis separations, e.g., desalination.
It is another object of the present invention to provide improved chemically modified polybenzimidazole semipermeable membranes which are suited for combined reverse osmosis/ion exchange separations or combined ultrafiltration/ion exchange separations.
It is a further object of the present invention to provide improved chemically modified polybenzimidazole semipermeable membranes which particularly are suited for use in separations from acid waste streams.
These and other objects, as well as the scope, nature, and utilization of the claimed process will be apparent to those skilled in the art from the following description and appended claims.